You’ve seen the ads: Valerie Bertinelli lost 40 pounds using the Jenny Craig diet program. Marie Osmond lost 50 pounds on Nutrisystem. Both plans rely on portion control through prepared foods delivered to your door, but do they work?

“These plans are best suited for those with no time to cook or grocery shop. They offer the ultimate convenience by delivering to the door preportioned, prepackaged food, so there’s no thinking involved,” says registered dietitian Shara Aaron, co-author of The Baby Fat Diet. “I’ve also seen these plans utilized to kick-start the weight loss. It’s appealing because it’s simple. Dieters know that what they’re eating is the right amount to lose weight.”

Nutrisystem bills itself as a portion-controlled, low-fat, reduced-calorie diet based on the glycemic index, which is a fancy way of saying the foods are high in good carbs and fiber. This combination helps dieters feel full longer. According to the company’s website, these good carbohydrates are digested slowly to ward off steep drops in blood glucose levels that can lead to food cravings.

Jenny Craig relies on portion control and one-on-one support. Participants begin the process by meeting with a program director to discuss their current eating habits, activity level and weight-loss mindset. Dieters then meet with a consultant each week — either in person or by phone — to review their progress.

Both programs stress the importance of exercise in conjunction with the meal plans and both offer tools, community forums and phone- and Web-based support to help participants develop healthier lifestyles. However, the weight loss comes with a hefty price tag. Nutrisystem plans start at about $300 for 28 days’ worth of food, including breakfast, lunch, dinner and dessert. Jenny Craig programs start at about $360 per year plus the cost of food. Food prices vary, but generally, participants spend $12 – $18 per day.

What’s more, the weight loss may be short-lived. “These weight-loss programs have a mass appeal, as they are a quick and easy fix, but generally, they are not effective for the long term,” says Kelly Sennholz, founder and chief medical officer for Symtrimics, a wellness software platform. “When the diet is over, people go back to old habits and regain weight.”

Michael Aziz, an attending physician and the author of The Perfect 10 Diet, agrees. “They are easy programs. They work in the short term, but many find it difficult to live on packaged food for the rest of their lives and gain the weight back.”

The food choices and available support are the most important factors to consider. Will you eat the meals offered? How healthy are the meals? Can you stick to the schedule? When dieters sign up, they can create a custom menu from dozens of options for each meal — and the options are diverse. However, among the offerings, some are healthier than others — and taste is certainly a factor. “These programs with packaged foods have a lot of salt and added chemicals, such as high-fructose corn syrup and hydrogenated fats, so beware of the food choices given. Read the labels,” Aziz says.

Aaron adds: “These plans can be just as nutritionally balanced as following a healthy plan like the USDA pyramid, but there may be a lot of preservatives and sodium, and natural sources of fiber may be lacking. And eating those prepackaged meals all the time can get very old very fast. After a number of months on the plans, dieters become bored or real life starts to set in, and they need a way to either continue losing weight or maintain their loss.”

Each dieter is different, but one tenet holds true: Long-term success requires big lifestyle changes, and those changes may not come automatically with weight-loss programs that offer packaged foods. “Good health is not a quick fix. It is a process that, for many, must be learned and practiced,” Sennholz says.